In photolithography, one step in the production of a microdevice, use is made of an exposure apparatus for projection exposure of images of patterns of a photomask or reticle (hereinafter referred to generally as a “reticle”) on to a substrate for exposure (semiconductor wafer or glass plate coated with a photoresist, light-transparent substrate called a “blank”, etc.) In recent years, to deal with the increasingly large size of the exposure area accompanying the increased size of substrates, a stitching type exposure apparatus which partitions the exposure area of the substrate into a plurality of unit areas (hereinafter sometimes referred to as “shots” or “shot areas”) and successively projects and exposes images of corresponding patterns on the shots has been developed.
In such an exposure apparatus, there was sometimes misalignment in stitched portions of shots due to aberration of the projection optical system, positioning error of the reticle or substrate, etc. Therefore, parts of the image of the patterns for one shot were superposed over parts of the images of the patterns for other shots adjoining it in the exposure. At an overlay parts of images of patterns, the exposure becomes greater than parts other than overlay parts, so for example the line width (width of lines or spaces) at overlay parts of patterns formed on the substrate becomes thinner or thicker in accordance with characteristics of the photoresist.
Therefore, the profile of exposure at parts forming overlay parts of the shots is set to a slant so as to become smaller the further toward the outside and the overall amount of exposure of overlay parts is made equal to the exposure of parts other than overlay parts by two exposures so as prevent changes in line width at these overlay parts.
As a technique for realizing a slanted profile of exposure at overlay parts of shots, it is known to form light-attenuating parts limiting in a slanting fashion the amount of light transmittance at portions of the reticle itself corresponding to overlay parts. Due to the formation of the light-attenuating parts in the reticle itself, however, the steps and cost of the manufacturing process of the reticle increase and the cost of manufacturing the microdevice etc. increase. Therefore, an exposure apparatus is being developed which is provided with a density filter formed with light-attenuating parts similar to the above on a glass plate at positions substantially conjugate with the pattern formation surface of the reticle or which is provided with a blind mechanism having light-blocking plates (blinds) able to advance into or retract from the optical path at positions substantially conjugate with the pattern formation surface of the reticle and realizes a slanted profile of exposure by making the light-blocking plates advance or retract during the exposure of the substrate.
When using the above-mentioned exposure apparatus to transfer the patterns of a reticle to a substrate, however, it is required that the pattern density dependency of ΔCD (OPE characteristic: optical proximity effect) be small. Here, “ΔCD” means the amount of deviation of the patterns to be formed on the substrate with respect to the target line width (line width error). The “OPE characteristic” means the characteristic of change of the ΔCD, even when the line widths of the patterns on the reticle are the same, depending on whether the patterns are isolated patterns or dense patterns and also whether they are lines or spaces. Further, the degree of change of the ΔCD does not only depend on the density of the patterns and is known to also depend on the wavelength of the illumination light irradiated on the reticle, the numerical aperture (NA) of the projection optical system, the pattern size, the illumination σ (σvalue=emission side numerical aperture of illumination system/incident side numerical aperture of projection optical system), etc.
Here, an example of the OPE characteristic of an exposure apparatus provided with a KrF excimer laser (wavelength: 248 nm) light source and provided with a 0.75 numerical aperture projection optical system will be explained. FIG. 14 is a view of the OPE characteristic when changing the density of patterns with a pattern size of 360 nm and illumination σ. FIG. 15 is a view of the OPE characteristic when changing the density of patterns with a pattern size of 200 nm and illumination σ. The “pattern size” spoken of here is the size (line width) of the patterns on the substrate. In FIG. 14 and FIG. 15, the abscissa gives the ratio of the lines and spaces and the ordinate gives the amount of deviation (ΔCD) from the target line width.
Referring to FIG. 14, it will be understood that when the pattern size is 360 nm, the amount of deviation from the target line width when changing the ratio of lines and spaces tends to deteriorate as the patterns become isolated lines except when setting the illumination σ to 0.55. Further, from the results of FIG. 14, it will be understood that when setting the illumination σ to 0.55, the extent of the amount of deviation from the target line width when changing the density of the patterns (extent of ΔCD) becomes the minimum, so the optimal illumination σ for the pattern size (360 nm) is 0.55.
Next, referring to FIG. 15, it will be understood that when the pattern size is 200 nm, the amount of deviation from the target line width when changing the ratio of lines and spaces tends to deteriorate as the patterns become isolated lines regardless of the setting of the illumination σ. Further, in the results shown in FIG. 15, the extent of the amount of deviation from the target line width (extent of ΔCD) when changing the density of the patterns becomes the minimum when setting the illumination σ to 0.85, so it will be understood that the illumination σ optimal for the pattern size (200 nm) is 0.85. From the above, to improve the OPE characteristic (pattern density dependency of ΔCD), it is necessary to set the optimal illumination σ for each pattern size.
However, if for example changing the illumination a for each pattern size of the patterns for transfer in order to improve the above OPE characteristic, the widths of the overlay parts of the shots explained above will end up changing. The reason is that when using a density filter having light-attenuating parts changing the probability of presence of fine dots for example in order to obtain a slanted profile of exposure at the overlay parts of the shots, sometimes the fine dots are prevented from being resolved by arranging the density filter off from the conjugate plane of the pattern forming surface of the reticle. At the time of such an arrangement, if changing the illuminations, the incident angle of the light rays on the density filter will end up changing and as a result the width of the overlay parts of the shots will change.
FIG. 16 is a view of the changes in the profile of exposure when changing the illumination σ, while FIG. 17 is a view of the amount of exposure at the overlay parts when changing the illumination σ. In FIG. 16 and FIG. 17, PR0 is the profile of exposure when setting the illumination σ at the standard value. Further, W is the width of an exposure area (width of transfer of patterns) when setting the illumination σ at the standard value, while WO is the width of an overlay part. Further, PR1 is the profile of exposure when reducing the illumination σ, while PR2 is the profile of exposure when increasing the illumination σ.
As will be understood from FIG. 16 and FIG. 17, if reducing the illumination σ, the width of an overlay part changes by being reduced from W0 to W1 and the gradient of the amount of exposure at the overlay part becomes sharp. Therefore, if even a slight offset in position of the shots occurs, the amount of exposure of the overlay part for the offset will change greatly and as a result the change in line width will become sensitive to the offset. Conversely, if increasing the illumination σ, the width of the overlay part will change by being enlarged from W0 to W2 and the gradient of the amount of exposure at the overlay part will become gentler, so the change of the amount of exposure of the overlay part for offset of the shots will become smaller. However, part of the overlay part will be enlarged outside of the exposure area and that part outside of the exposure area will end up being blocked by a light-blocking strip formed at the reticle.
As shown in FIG. 17, when setting the illumination σ at the standard value, the combined amount of exposure PR10 at the overlay part W0 will become the same as the amount of exposure outside of the overlay part. Even if the illumination His reduced, the combined amount of exposure PR11 at the overlay part W1 will become the same as the amount of exposure outside of the overlay part W2. However, if increasing the illumination σ, locations will arise (locations shown by the symbol Q in FIG. 17) where the combined amount of exposure PR12 in the overlay part W2 differs from the amount of exposure outside the overlay part, so the line width will change in the overlay part W2.
In this way, in the past, there was the problem that if for example changing the illumination σ or other illumination conditions to improve the OPE characteristic, the amount of change in the combined amount of exposure of an overlay part with respect to offset of the shots became greater and there was a susceptibility to changes in line width and also locations arose where the combined amount of exposure in the overlay part became insufficient and changes in line width occurred in the overlay part.